1. Field of the Invention
The present invention concerns high voltage switches and, more particularly, electric arc discharge or spark gap type switches.
2. Description of the Prior Art
High voltage switches capable of providing interruptable current paths for voltages in excess of several kilovolts have a number of diverse applications. These switches are also particularly useful if they further have the capacity to handle large current loads as well. Switches of this type are commonly used, for example, to discharge high energy capacitor banks and switch on various types of high power lasers. Because of the high voltages and, in some applications, high currents involved, solid state electronic devices, such as power MOSFETS and the like, are seldom capable of providing an adequate switching mechanism. Similarly, electro-mechanical devices, such as relays, are also of limited use since they are subject to electrical shorting and/or contact fusing (causing a permanent "switch-on" condition) if the operating voltages and currents are high enough.
One type of switching device commonly employed for very high voltages and/or high voltage-high current applications is the electric arc discharge or "spark gap" type switch. Spark gap switches typically include some sort of gas-tight body containing an inert gas and a pair of rugged switching electrodes mounted in opposing sides of the body structure. In operation, the voltage across the electrodes is raised above the electrical breakdown potential of the gas, causing an arc to form between the electrodes through the gas. An electrical current then flows through the switch until the source voltage to the switch is dissipated. Alternatively, an electrical probe may be positioned between the electrodes with the discharge arc initiated by raising the voltage between the probe and one of the electrodes above the breakdown potential for the gas.
While very useful for a diverse number of high voltage switching applications, spark gap switches suffer from several disadvantages. If a probe is employed to initiate the discharge arc, the switching circuit for raising the voltage of the probe must be isolated from the high voltage source of the electrodes in order to prevent shorting between the probe circuitry and the high voltage source. In spark gap switches which do not employ a probe, it is sometimes desirable, though generally not possible, to activate the switch at voltages below the breakdown voltage of the gas contained in the switch. In addition, the discharge arc or "spark current" present between the electrodes during activation of the switch typically causes erosion of the electrode material. This erosion usually affects the voltage at which the spark gap switch turns on by altering the shape of the electrodes and by plating the interior of the switch body with metal removed from the electrodes. The electrode material plating provides an alternate current path through the switch body which, over time, has a resistance much lower than the resistance of the gas contained in the switch, causing the switch to turn "on" at an undesirably lower voltage. Consequently, spark gap type switches must be periodically disassembled and the electrode plating removed from the interior of the switch body or the "switch-on" voltage may become erratic and undesirably low.
Thus, there exists a need for an improved spark gap type switch which is less susceptible to electrode erosion and more easily activated over a range of voltages.